CHAPTER 8

ENGINE DRIVE MECHANISMS Frequently, the source of power that operates one engine part is also the source of power for other parts and accessories of the engine. The source of power that operates engine valves may also be the source of power that operates such items as the governor; fuel, lubricating, and water pumps; and overspeed trips. Since mechanisms that transmit power to operate specific parts and accessories may be related to more than one engine system, we will discuss drive mechanisms before getting into the engine systems. After reading the information in this chapter, you should be able to recognize the basic design, function, and arrangement of various parts associated with drive mechanisms of 2-stroke and 4-stroke cycle diesel engines.

LEARNING OBJECTIVES When you have completed this chapter, you will be able to do the following:

1. Identify the drive mechanism for a 2-stroke cycle, in-line diesel engine. 2. Identify the drive mechanism for a 2-stroke, V-type diesel engine. 3. Identify the drive mechanism in an opposed-piston engine. 4. Identify the drive mechanism in a 4-stroke cycle diesel engine.

DRIVE MECHANISMS FOR A 2-STROKE CYCLE, IN-LINE DIESEL ENGINE Drive mechanism identifies the group of parts that takes power from the crankshaft and transmits that power to various engine components and accessories. The drive mechanism does not change the type of motion, but it may change the direction of motion. For example, the impellers of a blower are driven or operated by a rotary motion from the crankshaft transmitted to the impellers by the drive mechanism, an arrangement of gears and shafts. While the type of motion (rotary) remains the same, the direction of motion of one impeller is opposite that of the other impeller as a result of the gear arrangement within the drive mechanism. A drive mechanism may be a gear, chain, or belt type. The gear type is the most common. Some engines use chain assemblies or a combination of gears and chains as the driving mechanism. Belts are not common on marine engines, but are used as drive mechanisms on gasoline engines. Some engines have a single-drive mechanism that transmits power to operate engine parts and accessories. In some engines, there may be two or more separate mechanisms. When separate assemblies are used, the one that transmits power to operate the accessories is called the accessory drive. Some engines have more than one accessory drive. A separate drive mechanism that serves to transmit power to operate engine valves is generally called the camshaft drive or timing mechanism. The camshaft drive, as the name implies, transmits power to the camshaft of the engine. The shaft, in turn, transmits the power through a combination of parts which causes the engine valves to operate. Since the valves of an engine must open and close at the proper moment (with respect to the position of the piston) and remain in the open and closed positions for definite periods of time, a fixed relationship must be maintained between the rotational speeds of the crankshaft and the camshaft. Camshaft drives are designed to maintain the proper relationship between the speeds of the two shafts. In maintaining this relationship, the drive causes the camshaft to rotate at crankshaft speed in a 2-stroke cycle engine and at one-half crankshaft speed in a 4-stroke cycle engine.

Figure 8-1 — Camshaft and accessory drive.

There is considerable variation in the design and arrangement of the parts of drive mechanisms found in different engines. The size of an engine, the cycle of operation, the cylinder arrangement, and other factors govern the design and arrangement of the components as well as the design and arrangement of the mechanisms. Some of the variations in drive mechanisms are considered in the descriptions and illustrations that follow. The arrangements of the drive mechanisms described in this chapter are represented of those commonly found in marine engines used by the Navy. In some engines, the operating mechanisms consist of a single-drive mechanism. A complete assembly that transmits power from the driving part to the driven part, the operating mechanism consists of gears, shafts, and couplings.

Gears When the driving mechanism of an engine consists only of gears, the mechanism is commonly called a gear train. In a gear train, the gears must be accurately cut and heat-treated to resist wear. Helical teeth (teeth placed at an angle) are frequently used in place of spur teeth (teeth placed straight) for greater quietness and more uniform transmission of power. Gears and shafts are used in various arrangements to drive the vital components and accessories of the engine. The arrangement shown in Figure 8-1 is designed for right-hand rotation, and it functions as both the camshaft drive and the accessory drive. The train consists of five helical gears completely enclosed at the rear end of the engine. Note that all gears are driven by the crankshaft gear through an idler gear. An idler gear is placed between two other gears to transfer motion from one gear to the other without changing their direction. In Figure 8-1, the idler gear may be located on either side of the engine, depending upon the direction of the crankshaft’s rotation, (locate the spacer, or “dummy hub”). The use of a single idler gear is shown in Figure 8-2.

Since the engine operates on a 2-stroke cycle, the camshaft and balancer gears are driven at the same speed as the crankshaft gear. Either the camshaft gear or the balancer gear may be driven by the crankshaft gear through the idler gear; the drive arrangement depends on the model (right or left-hand rotation). The camshaft and balance shaft gears are counterweighted for balance purposes. The accessories receive power from the blower drive gear, which is driven by the camshaft gear (Figure 8-1). Located on the blower side of the engine and supported by the rear end plate, the blower drive gear transmits power to the blower, governor, water pump, and fuel pump, as shown in Figure 8-3. Figure 8-3 also shows the location of the various engine accessories and the shafts, gears, and couplings that transmit the power from the blower drive gear to each of the accessories.

Figure 8-2 — Idler gear.

Figure 8-3 — Blower, drive assembly, and accessories. Mechanical governor attached.

The blower end of the governor drive shaft is serrated or splined, and it engages with corresponding serrations or splines inside the upper blower shaft. The fuel pump is bolted to the rear cover of the blower and is driven from the lower blower rotor shaft through a device that acts as a universal joint. The water pump is mounted on the front end of the blower and is driven by the rotor shaft, through a coupling (Figure 8-3).

Figure 8-4 — Front gear train.

Figure 8-5 — Rear gear train.

DRIVE MECHANISMS FOR A 2-STROKE CYCLE, V-TYPE DIESEL ENGINE The in-line engine discussed in the preceding section requires only one drive mechanism (gear train) to transmit power to the valve actuating gear and engine accessories. Our discussion will now cover an engine that uses two separate gear drives (gear trains), one at each end of the engine. The front gear train, shown in Figure 8-4, consists of a crankshaft gear and two idler gears. The idler gears serve to drive the water pump (not shown) and balance the engines. (See balance weights.) The rear gear train (Figure 8-5) consists of a crankshaft gear, three idler gears, and two camshaft gears. The rear idler gears, like the front, also serve to balance the engine. (See balance weights.) The two other gears that are mounted on the rear of the engine, as shown in Figure 8-5, are the accessory drive gear and the blower drive gear. The correct relationship between the crankshaft and the two camshafts must be maintained so that the fuel injection, the opening and closing of exhaust valves, and the engine balance can be properly controlled. Since the camshaft must be in time with the crankshaft, timing marks are stamped on the face of the gears to facilitate correct gear train timing. The timing marks stamped on various gears are shown in Figures 8-1 and 8-6. When an engine is assembled, whether it is a 2-stroke or 4-stroke cycle engine, it is important that the appropriate timing marks be lined up on the gears as each gear is installed.

DRIVE MECHANISMS IN AN OPPOSED-PISTON ENGINE The drive mechanisms of an opposed-piston engine will obviously differ, to a degree, from those of single-acting engines because of design differences. Some of the differences are because:

• Power is supplied by two crankshafts in an opposed-piston engine, instead of one.

• The camshaft drives of the engines we have discussed thus far supply power to one or more accessories as well as to the valve-actuating gear. This is not true of the camshaft in an opposed piston engine, since ports are used instead of valves for both intake and exhaust.

Figure 8-6 — Rear gear train timing marks.

Figure 8-7 — Location of drive mechanisms in an opposed-piston engine (Fairbanks-Morse 38D8 1/8).

Regardless of differences in mechanisms, the basic types of drives, gear and chain, are found in both single-acting and opposed piston engines. While the two engines described in preceding sections had only gear-type drive mechanisms, the opposed-piston engine used as an example in this section has chain assemblies as well as gear trains incorporated in the mechanisms that supply power to engine parts and accessories. The Fairbanks-Morse (FM) opposed-piston engine has three separate drive mechanisms. The drive that furnishes power to the camshaft and fuel-injection equipment is the chain type. The blower and the accessories are operated by gear type drives. The location of each drive is shown in Figure 8-7.

Camshaft Drive-Actuating Gear The opposed-piston engine does not have cylinder valves; since two other drives are provided to operate the accessories, the primary purpose of the camshaft drive is to transmit power for, and to time the operation of, the fuel injection pumps. The camshafts are located in the upper crankshaft compartment (Figure 8-7). The shafts turn at the same rate of speed as the crankshaft.

Chain Assembly The power required to operate the fuel injection pumps at the proper instant during the cycle of operation is transmitted through the camshafts from the crankshaft by a chain drive (frequently called the timing mechanism). The names and arrangement of the components of the drive are shown in Figure 8-8. The drive sprocket is attached to the upper crankshaft at the control end of the engine. A sprocket is attached to the end of each camshaft, and there are three other sprockets for timing and adjustment purposes. The chain conveys the rotation of the upper crankshaft to the camshaft sprockets by passing over the crankshaft sprocket, under the two timing sprockets, over the two camshaft drive sprockets, and under the tightened sprockets. The timing sprockets are mounted on an adjustable bracket or lever. By moving the lever, the timing of the two camshafts can be adjusted. The adjustable tightened sprocket is used to obtain and maintain the proper slack in the chain.

Blower Drive Mechanism The power to drive the blower is transmitted from the upper crankshaft, through a gear train (Figure 8-7). The train consists of a drive gear, a pinion gear, and the two timing (impeller) gears of the blower. The drive gear is the flexible type (Figure 8-7). The principal parts of the flexible drive gear are a spider drive hub (which is keyed to the crankshaft), a gear (within which spring spacers are bolted), and springs (which absorb torsional oscillations transmitted by the crankshaft). The flexible drive gear with end plate removed and the spider drive hub is shown in Figure 8-9. The flexible drive gear meshes with the drive pinion (Figure 8-7). The pinion is keyed to the lower impeller shaft and held in place by a locknut. The lower impeller driving (timing) gear meshes with the upper impeller driven gear (Figure 8-7).

Accessory Drive Mechanism The majority of the accessories for the FM 38D8 are driven by a gear mechanism that receives power from the lower crankshaft at the control end of the engine (Figure 8-7). A more detailed view of the accessory drive is shown in Figure 8-10. Referring to both these figures as you read the following description will help you become familiar with the components of the drive and with the way that power is transmitted to the driven units.

Figure 8-8 — Camshaft drive and timing mechanism.

Figure 8-9 — Blower flexible drive gear.

Figure 8-10 — Accessory drive (Fairbanks-Morse 38D8 1/8).

The accessory drive transmits power to the water pumps, the fuel oil pump, the lubricating oil pump, and the governor. The drive gear (Figure 8-7) of the mechanism is bolted to a flange on the crankshaft. The drive gear is the flexible type; therefore, engine shocks transmitted by the crankshaft are absorbed by the drive springs of the gear. The water pump drive gears mesh directly with the flexible drive gear. The fuel pump drive gear (attached to the flexible drive gear) transmits power to the fuel pump driven gear through an idler, the fuel pump driven gear on the mounting plate in Figure 8-10. The lubricating oil pump drive gear meshes directly with the flexible drive gear. Power is transmitted to the pump through a shaft and an internal gear coupling—the lubricating oil pump drive. The shaft of the lubricating oil pump drive also transmits power to the governor. A gear on the shaft meshes with a mating gear on the governor drive gear shaft. This shaft drives the governor coupling shaft which, in turn, drives the governor, through a beveled gear drive (Figure 8-7).

Figure 8-11 — Gear train of a 4-stoke cycle engine showing gear ratio.

DRIVE MECHANISMS IN A 4-STROKE CYCLE DIESEL ENGINE The drive mechanisms we have discussed so far have applied to 2-stroke cycle engines. We will now take a look at a gear train of a 4-stroke cycle engine (Figure 8-11). The gear train for the 4-stroke cycle engine is different from that of the 2-stroke cycle engine for two reasons. The first reason is that there is no provision for the driving of a blower since 4-stroke cycle diesel engines are either naturally aspirated or are turbocharged. Turbocharging units are exhaust driven and require no mechanical drive. Another difference is the need for gear reduction in a 4-stroke cycle engine between the crankshaft and the camshaft. In a 4-stroke cycle engine, the camshaft speed must be exactly one-half the crankshaft speed. Remember, in a 4-stroke cycle engine, the crankshaft must rotate 720° for each power event per cylinder. The method by which the required 2:1 gear reduction is accomplished is by the use of one or more idler gears between the crankshaft gear and the camshaft gear. Refer to Figure 8-11 as we explain how the 2:1 gear ratio is obtained in one type of engine. If the crankshaft were to revolve 360 degrees, it would move a total of 48 teeth. This movement is transmitted to the large idler gear, which will also revolve 360 degrees, or 48 teeth. As you can see in Figure 8-11, a smaller idler gear with 30 teeth is mounted to the larger idler gear. This smaller gear is where the gear ratio starts to change. As the small idler gear revolves 360 degrees, or 30 teeth, it drives the camshaft gear 180 degrees, or 30 teeth. (The gear with the greater number of teeth will always revolve more slowly than the gear with the smaller number of teeth.) Thus, we now have the 2:1 ratio between the crankshaft and camshaft that is required for a 4-stroke cycle engine to operate. Now compare the gear train of the 4-stroke cycle engine in Figure 8-11 to that of the 2-stroke cycle gear train in Figures 8-1 and 8-5. You should be able to recognize the difference in these drive mechanisms.

SUMMARY The drive mechanisms of an engine are those assemblies that transmit power for the operation of engine accessories and certain engine parts. The drive mechanisms may be a chain assembly, gear train, belts, or a combination of any of these parts. Some engines have only one drive mechanism, which is generally called the camshaft drive. Other engines may have a second major drive mechanism called the accessory drive. In most engines, each drive is generally identified by the name of the principal part or the accessory drive, such as blower drive, camshaft drive, and governor drive. The importance of these drive-transmitting devices is evident if you consider the function of the components to which power is transmitted. The valve–actuating mechanisms control the fuel, intake air, exhaust gases, and starting air (when applicable) in the cylinders. The engine accessories that are driven are those that circulate the cooling water and lubricating oil, supply air for scavenging and supercharging, supply fuel, and control engine speed. If you are unsure as to how these drive mechanisms function, we recommend you review this chapter again.

End of Chapter 8

Engine Drive Mechanism

Review Questions 8-1. What is the drive design for the camshaft to rotate at relative to the crankshaft speed on a 4-

stroke cycle diesel engine?

A. Same speed as the crankshaft B. Twice the speed of the crankshaft C. Half the speed of the crankshaft D. Three times the speed of the crankshaft

8-2. Of the following variation in design and arrangements of drive mechanisms found on diesel

engines, which one is NOT considered?

A. Size of engine B. Cycle of operation C. Cylinder arrangement D. Number of cylinders

8-3. What kind of teeth is frequently used for more uniform transmission of power in drive

mechanisms?

A. Helical B. Herring bone C. Spur D. Straight

8-4. Why do the camshaft and balance gear use counterweights in the drive mechanism on the 2-

stroke in-line diesel engine?

A. Maintaining proper timing B. Balancing the camshaft and balancer gear with the crankshaft C. Balancing the camshaft with blower gear D. Balancing the idler and accessories gears with the camshaft gear

8-5. What component in the drive mechanism of a 2-stroke cycle diesel engine transmits power to

the blower, governor, water pump, and fuel pump?

A. Camshaft gear B. Blower driver gear C. Fuel pump gear D. Lube oil pump gear

8-6. What component is driven by idler gear in the front gear train of 2-stroke cycle V-type engine?

A. Fuel pump B. Lube oil pump C. Start air solenoid D. Water pump

8-7. How would someone know the correct way to place the gears on the gear train for the crankshaft and camshaft?

A. Wear patterns of old gears B. Reading the technical manual C. Timing marks D. All gears are universal

8-8. Of what medium is the drive constructed that furnishes power to the camshaft and fuel

injection equipment on an opposed-piston diesel engine?

A. Belts B. Chains C. Gears D. Wire rope

8-9. Where does the power to drive the blower come from on an opposed-piston diesel engine

drive mechanism?

A. Accessory drive B. Camshaft C. Lower crankshaft D. Upper crankshaft

8-10. Which of following does NOT receive power from the accessory drive gear on an opposed-

piston diesel engine drive mechanism?

A. Fuel pump B. Governor C. Start air solenoid D. Water pump

8-11. What must a 4-stroke cycle engine’s camshaft speed be, compared to the engines crankshaft’s

speed?

A. The same B. Twice C. Half D. Four times

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